The present invention relates to a method for making a gradiometer having a three-dimensional and structure and associated connecting lines for a device for the single or multichannel measurement of magnetic fields with field intensities down to less than 10.sup.-10 T and in particular below 10.sup.-12 T wherein the superconducting gradiometer coils of predetermined dimensions are arranged on a support body and are connected by means of superconducting connecting lines and a coupling coil to a cuperconducting quantum interference element (SQUID) located on a substrate body of its own in a magnetically coupling manner. Such a method is indicated in "Review of Scientific Instruments", Vol. 53, No. 12, December 1982, pages 1815 to 1845.
The use of superconducting quantum intereference elements which are generally called "SQUIDs" (abbreviation for "superconducting quantum interference devices") for measuring very weak magnetic fields is generally known ("J. Phys. E: Sci. Instrum.", Vol. 13, 1980, pages 801 to 813; "IEEE Transactions on Electron Devices", Vol. ED-27, No. 10, October 1980, pages 1896 to 1908). As a preferred field of application for these elements is therefore also considered medical technology, especially magneto-cardiography and magneto-encephalography, where magentic hear or brain waves with field strengths in the order of 50 pT or 0.1 pT respectively occur ("Biomagnetism-Proceedings of the Third International Workshop on Biomagnetics, Berlin, 1980", Berlin/New York, 1981, pages 3 to 31, or the publication "Rev. Sci. Instrum" mentioned above).
A device for measuring such biomagnetic fields contains essentially the following components:
(1) a SQUID as the essential field sensor,
(2) a flux transformer with a so-called gradiometer for coupling the biomagnetic flux into the transformer as well as a coupling coil for coupling the magnetic flux into the SQUID,
(3) electronic equipment for picking up and processing signals, PA1 (4) shields for the earth's magnetic field and external interference fields, and PA1 (5) a cryo system for ensuring superconduction of the sensor and the gradiometer.
The design and operation of such single-channel devices are known. In these devices, the magnetic field to be detected, which is smaller by up to 6 orders of magnitude than external interference fields, is coupled inductively, generally via a three-dimensional coil arrangement, into the circuit formed by an RF SQUID with a Josephson contact. Coil systems called first and higher-order gradiometers are designed by the combination of a sensor coil (also called detection coil) with one or more compensation coils. With such gradiometers, the three components of a magnetic field homogenous in the region of the coils or also its share with homogeneous gradients can largely be suppressed with appropriate manual balancing and the biomagnetic near field which is still heavily non-uniform in the region of the gradiometer can be picked up selectively.
In order to obtain with such a device three-dimensional field distribution, measurments must be performed sequentially at different locations of the region to be examined. The difficulty arises however, that during the measuring time required therefor, the coherence of the field data is no longer assured and, in addition, clinically insufferable measuring times result. It has therefore been proposed to make a multi-channel measurement instead of the known single-channel measurement (see, for instance, "Physica", Vol. 107B, 1981, pages 29 and 30). Here, every channel has, besides an RF-SQUID, a tunable superconducting gradiometer and interlinking elements between the SQUID and the gradiometer with a coupling coil calso called a coupling transformer and with connecting conductors. In such a device, however, a considerable time-consuming effort is necessary with respect to the tuning of the individual channels to each other. This is because the gradiometer on the one hand and the SQUID with its coupling coil on the other hand are arranged respectively on separate support bodies, according to the known device, where these parts can be connected via detachable connecting conductors. However, constant tuning of the respective flux transformer cannot be taken for granted with such a connecting technique. Rather, before every measurement, an adjustment of all channels is necessary which also influence each other. Also, in such an arrangement, the mechanical stability of the once tuned gradiometers is relatively low so that the adjustment can readily be cancelled again by mechanical influences. A further disadvantage of the conventional gradiometers is due to the fact that they are made of wire which is wound on a solid body. The mechanical hold of the gradiometer wire coil on the solid body is so small that an adjustment made at room temperature is not preserved when cooling down to the temperature of liquid helium. In addition, mutual interference of the RF circuits is unavoidable. While the mutual interference of the channels in an adjacent arrangement as well as the intrinsic noise of the individual channel can be reduced by the use of d-c SQUIDs instead of RF Squids (see, for instance, "IEEE Transactions on Magnetics", Vol. MAG-19, No. 3, May 1983, pages 835-844), it still is difficult to control the adjustment of the individual channels of a corresponding modular multichannel gradiometer system.
The three-dimensional structured gradiometers of the known devices are in general made of superconducting wire on appropriate coil forms, where manufacturing-related adjustment tolerances of about 10.sup.-3 can hardly be improved. An improvement of these tolerances is achieved by a subsequent mechanical adjustment. With this method, however, a realization of complex gradiometer rows, also called gradiometer arrays such as are required for multichannel measuring devices, is difficult to achieve since a mechanical adjustment is practically impossible to carry out in such devices.
In addition, it is known from DE-OS No. 32 47 543 to fabricate such gradiometer arrays by thin-film planar technology. While this technique permits a better adjustment and the realization of more complex structures, it is assumed however that the SQUID's of all channels are generated in the same plane and, in addition, the gradiometers come to lie in this plane. In the planar gradiometers obtained in this manner, while they are assembled to form higher-order three-dimensional gradiometers, the superconducting connecting technique required therefor for connecting the gradiometers to each other and to the coupling coils or the SQUIDs is very costly. In addition, also mechanical stability problems can result in such higher-order gradiometers.